[PATCH 5/7] btrfs: fix dirtied pages accounting on sub-page writes

[Wu Fengguang <fengguang.wu@intel.com>]

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When doing 1KB sequential writes to the same page,
balance_dirty_pages_ratelimited_nr() should be called once instead of 4
times, the latter makes the dirtier tasks be throttled much too heavy.

Fix it with proper de-accounting on clear_page_dirty_for_io().

CC: Chris Mason <chris.mason@oracle.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 fs/btrfs/file.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- linux-next.orig/fs/btrfs/file.c	2011-11-17 20:13:47.000000000 +0800
+++ linux-next/fs/btrfs/file.c	2011-11-17 20:18:51.000000000 +0800
@@ -1136,7 +1136,8 @@ again:
 				     GFP_NOFS);
 	}
 	for (i = 0; i < num_pages; i++) {
-		clear_page_dirty_for_io(pages[i]);
+		if (clear_page_dirty_for_io(pages[i]))
+			account_page_redirty(pages[i]);
 		set_page_extent_mapped(pages[i]);
 		WARN_ON(!PageLocked(pages[i]));
 	}